Fact: when upgrading your braking system, you need to upgrade all four corners. Do not upgrade the front brakes only. This will actually decrease your overall braking performance in racing.
Caveat (all facts have them, right): we're talking about a street car in predominantly stock form here used for both street and track use. The principles carry through to racier cars, but the numbers change enough to yield a different outcome. We'll explore that at the end.
To understand why it is important to upgrade all four corners at the same time, whether it be pads, rotor diameter, or both, we can make the simple statement that the auto manufacturer balanced the braking force of the front and rear wheels to start with for the best braking performance and stability with the given parts. This balancing takes into consideration the car's weight, weight distribution, center of gravity, wheelbase, and weight transfer during hard braking conditions. Unless you make substantial changes to the car's weight distribution or center of gravity location, whatever brake upgrades you make, you want to keep the ratio of the braking forces the same as the stock setup.
To show the effect of changing front pads only, front rotors and pads only, or all four rotors & pads, we'll look at a series of calculations to show the net effect of these changes on the force applied at the road and tire interface.
First, we will assume a certain clamping force generated at the caliper based on the driver's pedal pressure, pedal leverage, master cylinder, and caliper piston size. We will assume this as a constant input to see what happens throughout the rest of the braking system by changing pads and rotor diameter. Of course, the actual braking force at the wheel is variable based on the pressure the driver applies to the brake pedal.
The net result of the calculations of our theoretical setup shows that we start with a stock system designed to have a 64% front and 36% rear distribution of force. By changing only the front pads to a high friction racing pad, this braking distribution changes to 71% front and 29% rear distribution. If the front rotors are upgraded in size and the racing pads are used, the distribution changes to 74% front and 26% rear. This is a dramatic change from the intended balance of the stock setup. One very noticable effect is that the car will be less stable under hard braking -- the back end will wiggle around requiring the driver to control it. With an out-of-balance setup like this, stopping your car with huge front brake upgrades would be like stopping a bicycle or motorcycle using only the front brake. It's very unstable (and nerve racking).
Another noticeable effect with a setup like this, where only the front has been changed, is significant front brake and tire wear, and little rear brake and tire wear. This is often misinterpreted as "the more front brake you have, the less you neeed in the rear." The rear system wears less, and therefore appears to be needed less. In fact, the reduced wear is not because the rear brakes are needed less, it is because they are used less. We will prove this a little further on with some numbers.